I'm not usually that useful here on /r/AskHistorians (more of a lurker than a commenter), but maybe I'll have a chance to put my English masters to use!!

Shakespeare's importance in writing was multitude. First was his influence on language. He is often cited as (supposedly) having the largest published vocabulary of any writer, mainly because he invented so many of the words that he used. A good example is assassination, which he adapted into a word meaning the act of political killing from the established term assassin.

Second was the scope of his audience. Due to the nature of Elizabethan theatre, audience's could be common or noble. The immediate consequence of this is that these work that we consider to be so great now were being directly delivered to the common man simultaneously. His works equalized audiences, and would be the basis of context for multiple classes.

Finally, Shakespeare's works were incredibly broad and varied, allowing him to address a multitude of concepts. He wrote comedies, tragedies, and histories almost equally, but each genre would contain elements of the others. Shakespeare was able to psychological examine all walks of life and reexamine many pieces of history (British and European), but do it in an extremely accessible way. More importantly, Shakespeare wrote with what we call "negative capability." This is a term used to describe how Shakespeare was able to transcend the context of his writing and largely divorce his own persona from his voice. This means that, despite the wealth of writing we have from the man, we have very little idea about how he felt about many of the issue he wrote about; he was able to write about them from so many convincing perspectives without betraying his real thoughts about any!!

I have studied Cervantes, although not in as great of detail. However, I can let you know some of the groundbreakingly important things about Cervantes.

As has been noted elsewhere in this thread, Cervantes wrote what is either the first novel or one of the first novels. In this regard, he was extremely influential, not only for how we look at novel-length prose today, but also in his own time. In fact, Don Quixote was so popular in its time that during Cervantes' life, homage sequels were being written about Quixote and Sancho. Copycat writing was not unusual at this time (indeed, it would prove to be the basis of many of Shakespeare's works, as well as his contemporaries), but Don Quixote was written in a point in modernity that it began to matter.

This leads to one of the more interesting parts of Cervantes' writing: he was one of the first writers to really engage his own "meta." One of the sequels (attributed to Avellaneda) was popular enough that Cervantes decided to react and write a second part to his own novel, designed to specifically reject the confederate sequel. In fact, not only did this reject other copycats, but it presupposed that not only had all of Spain read the first part, but all of the characters in the novel had also read the first part!! Cervantes creates a narrative where his characters can engage in the real word reactions to his previous work, while continuing the critiques and pastiche of previous Spanish literature that was the hallmark of his first part.

This meta-novel would become highly influential in Spanish language literature. There is an entire genre (widely popular in Central and South America) called "magical realism," which is greatly informed by meta-literature. Jorge Luis Borges, a pillar in post-modern writing, has numerous works of self-aware, meta-, and faux-journalistic pieces of fiction, including a short story about Don Quixote!! So Cervantes' influence is not just the revolutionary medium of his writing, but also the method of engaging his own context to innovate plots and themes.

I'm not certain this is a history question. /r/literature, maybe?